1. Field of the Invention
The present invention relates to an improved fusible element for a current-limiting fuse and, more particularly, to an improved fusible element for a current-limiting fuse capable of protecting a high-voltage circuit against faults, which faults may occur at either a higher phase-to-phase voltage or a lower phase-to-ground voltage. More specifically, the present invention relates to an improved fusible element for a high-voltage current-limiting fuse capable of effectively interrupting faults at either voltage, which fusible element will not generate more than a predetermined back voltage while interrupting faults at the lower voltage. The present invention is an improvement of the invention described and claimed in commonly assigned U.S. Application, Ser. No. 194,712, filed Oct. 6, 1980, now Pat. No. 4,359,708, in the name of the inventors hereof.
Current-limiting fuses are, in general, well known. Such fuses serve two functions. First, and in common with all fuses, a current-limiting fuse responds to fault currents or other over-currents in a circuit by interrupting the current to protect the circuit. Such response is brought about by the inclusion in the fuse of a fusible element made of a material which melts, fuses, vaporizes or otherwise becomes disintegral when the I.sup.2 t heating effect of the fault current therein exceeds some predetermined value, determined by the dimensions and material of the fusible element. Second, unlike other types of fuses--power fuses and cutouts, for example--current-limiting fuses limit the magnitude of the fault current to some maximum value while interrupting it.
The most common type of current-limiting fuse is the so-called silversand fuse. In such a fuse, the fusible element is intimately surrounded by a compacted fulgurite-forming medium, such as silica or quartz sand. A fulgurite is a silicon substance formed by the fusing or vitrification of the sand or other medium due to its absorption of high energy, such as that accompanying lightning or an electric arc. The fusible element is often a ribbon of a fusible metal, such as elemental silver or copper, which may be straight or curvilinearly wound (for example, in a helical or spiral configuration) within an insulative housing for the sand. The ribbon may contain a pluarality of holes or notches formed therethrough or therein which, in effect, decrease the cross-section of the ribbon at selected points. See U.S. Pat. Nos. 4,123,738, 4,204,183, 4,204,184, and 4,210,892; Canadian Patent No. 876,884 (July 27, 1971); and West German Auslegeschrift No. 1,193,154 (May 20, 1965).
For purposes of explaining the present invention, it is assumed that a current-limiting fuse of the prior art is connected in a circuit between an AC power source and a load powered by the source, the fusible element being in electrical series with the source and the load. The circuit may be one phase of an AC high-voltage, three-phase electrical system, each phase of which may include a similar fuse. The circuit may be viewed as also containing a single series inductance representative of all the inductance thereof "lumped" together.
The fusible element of each fuse is selected so that if the current driven through the circuit to its load by the source is "normal," that is, below a selected level, the heating effect of the current (I.sup.2 t) is insufficient to melt, fuse or vaporize the fusible element at any of the holes or notches where its cross-section is decreased. During the time normal current flows, there is a small, nearly zero, voltage drop across each fuse. If, for any reason, the current in any fuse exceeds the selected level for a sufficient time (e.g., due to an overload or fault), I.sup.2 t is sufficient to melt, fuse or vaporize its fusible element across the width thereof at the points of formation of the holes or notches.
At each melted location--which quickly assumes a pair of sites or fronts extending across the width of the fusible element, the sites being separated along the length of the ribbon--a gap is produced. An arc is established in each gap with its ends terminating on the separated sites or fronts. Each arc generates an arc voltage or back voltage opposite in polarity to the source voltage, the total arc voltage of the fuse being the cumulative or additive effect of the arc voltages of all the arcs so formed. Thus, if the fusible element has one hole or notch therein, an initial arc or back voltage V.sub.a is generated; if the fusible element has four similar holes or notches therein, an initial arc or back voltage 4 V.sub.a is generated; if the fusible element has N similar holes or notches, an initial arc or back voltage NV.sub.a is generated. Typically, the initial arc or back voltage of the fuse "jumps" or rises in a very short time from the small, nearly zero, normal voltage drop across the fuse to a substantial value which is initially somewhat less than the source voltage. This jump or rise in the fuse's arc voltage or back voltage occurs immediately after the arc or arcs form.
Each arc is both constricted and cooled by the compacted sand; both effects further elevate the arc or back voltage of the fuse. Constriction is the result of "forcing" each arc to traverse a path confined by the compacted grains of the sand which reside in and about each gap between the sites or fronts. Cooling of the arcs, which is due to "heat-sink" effect of the sand, absorbs energy therefrom, thereby forming the fulgurite.
Following their initial formation, the arcs "burn back" or melt away the element in opposite directions away from the former location of the holes or notches. The ends of each arc, and the respective opposed sites or fronts on which each terminates, constantly "move" away or recede from each other as each arc burns back the element to widen the gap in which it is formed. The "movement" of the sites or fronts away from each other elongates the arcs and exposes the ends of each arc to "fresh" or "new" sand. The "fresh" or "new" sand further constricts and further cools the elongating arcs. Thus, as long as the arcs persist and ribbon is available for consumption by the arcs, they continually elongate and have their elongating length further constricted and cooled. This results in yet further elevation of the arc or back voltage with time. Ultimately, the arc or back voltage of the fuse exceeds the source voltage. In sum, then, the arc or back voltage generated by the fuse depends on both the number of arcs formed and the amount of burn-back of the element by these arcs. The rate of burn-back is, in turn, related to the material and shape of the fusible element and to the level of current in the fusible element.
Shortly after the initial jump in arc or back voltage, which is followed by a continuing increase therein with time due to burn-back, the circuit current (which is out of phase with the circuit voltage due to the circuit inductance) begins to "turn down" or to be forced to continuously decreasing levels. As the current turns down, the arc voltage continues to increase, albeit at a slower rate, as the arcs continue to burn back the fusible element. The increasing arc voltage causes the current to continuously turn down or decrease. Assuming there to have been a sufficiently long fusible element with sufficient distance between the holes or notces, this process continues until the current is "turned down" to zero. At zero current in the fuse, the circuit is interrupted if the dielectric strength of the gaps is sufficiently high. The turn down is current shortly after the arc or back voltage begins to increase results in the fuse acting in a current-limiting or energy-limiting manner. That is, during the operation of the fuse, the circuit current assumes a lower maximum value--its value just prior to turn-down--than it otherwise would have assumed, thus protecting the circuit and devices connected thereto from excessive over-currents.
During the operation of a silver-sand current-limiting fuse, arc or back voltages in excess of the source voltage are generated. Indeed, it is necessary that the fuse's arc or back voltage exceed the source voltage for current limitation to occur. If the fusible element is very long, current interruption may be very effective, although very high arc or back voltages will be generated. As a result, typical current-limiting fuses include elements of reasonable lengths, that is, lengths selected so that the elements are nearly totally burned back or nearly consumed at a time when the turn-down in current is sufficient to assure that current zero will be reached. Should the fusible element be totally consumed, all the arcs merge into a single long arc, the arc or back voltage of which cannot further increase because no "fresh" or "new" sand can be introduced into the gaps. In typical fusible elements, the holes or notches are evenly spaced so that the fusible element is burned back the same amount between each hole or notch, and so that merger of all arcs into the single long arc takes place at the same time. Before merger, the number of arcs is equal to the number of holes or notches and the number of receding sites or fronts at which burn-back occurs is twice the number of holes. Thus, while typical current-limiting fuses operate prior to merger, the arc or back voltage thereof is simply equal to the product of the number of holes or notches initially present multiplied by the arc or back voltage of any one of the similar arcs. The arc or back voltage of the fuse increases as long as burn-back occurs and the rate of the arc or back voltage increase is equal to the product of the number of holes or notches multiplied by the rate of arc or back voltage increase of any one of the similar arcs.
In many circuits, faults may occur at a lower or a higher voltage. In a 15 kv (phase-to-phase voltage) three-phase AC circuit, for example, phase-to-phase fault currents are, in effect, driven by a 15 kv source voltage while phase-to-ground fault currents are driven by a phase-to-ground source of approximately 9 kv. If current interruption is the sole desideratum, a single fusible element can be chosen which will ensure interruption of fault currents at both voltages. Specifically, a fusible element sufficiently long to generate a very high arc or back voltage at either source voltage can be selected.
Thus, the fuse in each phase can be selected so that it is, by itself, capable of interrupting phase-to-ground fault currents which occur only in its phase and which are not "seen" by the other phases or the fuses therein. Care must be used, however, in selecting fusible elements which will not cause the operation of surge arrestors connected between each phase and ground. If the selected fusible element is "too long" or for any other reason generates an arc or back voltage which is "too high," the arc or back voltage of the fuse will ultimately exceed the surge arrestor voltage and cause sparkover thereof. Arrestors rated 9 kv (phase-to-ground voltage) will typically sparkover at about 25-27 kv. Thus, when the fuse interrupts phase-to-ground fault currents driven by a 9 kv source voltage, it is desirable that the arc or back voltage of the fuse not exceed 25-27 kv.
Even though each fuse by itself might not be capable of interrupting fault currents driven by the higher (15 kv) phase-to-phase voltage, such faults necessarily involve the fuses of the faulted phases in electrical series. Accordingly, the fuses are selected so as to be able, in a series combination, to interrupt the fault current by together generating a sufficiently high arc or back voltage.
From what has been said above, in typical current-limiting fuses the fusible element itself is the current-responsive "trigger" for the fuse. When current gets sufficiently high, the I.sup.2 t effect thereof initiates melting of the fusible element followed by current-interrupting operation of the fuse. This is true even in a phase-to-phase fault current situation where a fuse in one involved phase may operate before a fuse in another involved phase, due, for example, to normal manufacturing tolerances. Specifically, although one fuse may operate first and generate an arc or back voltage preventing the fault current from further increasing, the second fuse will, nevertheless, eventually operate because the element thereof responds to I.sup.2 t, not to I. That is, although I.sup.2 may not increase, the product of I.sup.2 and t will initiate operation of the second fuse when it becomes sufficiently large.
In a variant type of current-limiting fuse, a silver-sand fuse is shunted by a normally closed, high current-capacity switch. See commonly assigned U.S. Pat. No. 4,342,978, issued Aug. 3, 1982 in the name of Otto Meister; and commonly assigned U.S. patent applications, Ser. No. 188,660, filed Sept. 19, 1980, now U.S. Pat. No. 4,370,531, in the name of Thomas Tobin; Ser. No. 179,367, filed Aug. 18, 1980 (now abandoned in favor of continuation application Ser. No. 550,201, filed Nov. 9, 1983) in the names of John Jarosz and William Panas; and Ser. No. 179,366, filed Aug. 18, 1980 (now abandoned in favor of continuation application Ser. No. 539,396, filed Oct. 6, 1983) in the name of Raymond O'Leary. Because the switch has a high current-carrying ability, this arrangement permits the combination to have a very high continuous-current-carrying ability, which silver-sand fuses used alone do not have. The switch is opened by a current-sensor when the current reaches a value in excess of a selected level. The sensor responds to I or dI/dt, not to I.sup.2 t. When the switch opens, the current is entirely commutated to its fuse which begins to operate. As the fuse begins to operate, the fault current begins to decrease, as described above, whether the fault current is phase-to-phase or phase-to-ground. If, due to tolerance differences between the sensors associated with the fuses in two phases between which a fault current flows, only one sensor initially responds, the second sensor will not later respond because the fault current level is decreasing. Thus, only one fuse may be available to interrupt phase-to-phase fault currents, and its fusible element must be selected to achieve this end. Accordingly, each fuse must be capable of itself interrupting fault currents at the higher phase-to-phase voltage, assumed above to be 15 kv. As noted above, this can easily be achieved by appropriate selection of a fusible element. A problem arises, however, at lower voltage phase-to-ground fault currents where too long an element--that is, an element sufficiently long to interrupt phase-to-phase fault currents--is present.
Specifically, phase-to-ground fault currents commutated to the fuse by the opening of the switch cause the fusible element to melt at the holes or notches, as do the higher voltage phase-to-ground fault currents, and initiate burn-back of the fusible element at each site or front pair of either end of each arc. This action, as described above, effects the generation of the arc or back voltage. It has been found, however, that the arc voltage generated by a silver-sand current-limiting fuse, which by itself is capable of interrupting phase-to-phase fault currents, may well exceed the spark-over voltage of the phase-to-ground surge arrestors while interrupting phase-to-ground fault currents. Spark-over of the surge arrestors under the conditions described is undesirable, for arrestors are intended to protect the circuit in the event of surges such as those caused by lightning, and not by surges caused by current interruption by the fuse.
Commonly assigned U.S. patent application, Ser. No. 194,712, filed Oct. 6, 1980 now U.S. Pat. No. 4,359,708 discloses and claims a fusible element for a current-limiting fuse which interrupts fault currents driven by both higher phase-to-phase voltages and lower phase-to-ground voltages, while limiting the arc voltage generated by the fuse during interruption of fault currents at the lower voltage. Specifically, the fusible element comprises a conductive ribbon. A number of groups of holes or notches are formed through or in the ribbon. The groups extend single-file along the ribbon. Adjacent holes or notches of each group are spaced apart along the ribbon by a small distance. Adjacent groups are spaced apart along the ribbon by a distance substantially greater than the distance between the adjacent holes or notches within each group.
Faults occuring at higher phase-to-phase voltages melt the ribbon first at the reduced cross-sectional points thereof--that is, those locations where the holes or notches have been formed--and then burn back the ribbon between the groups until current interruption is effected. Lower phase-to-ground voltage fault currents first melt the ribbon at the hole locations, just as do the higher voltage fault currents. Because the distance between the holes within the groups is small, the numerous arcs formed first burn back the ribbon along the shorter distance between the holes and then the arcs of each group merge into a single arc. The ribbon is thereafter burned back between the groups by the merged arcs at a more gradual total rate than occurred before the merger because of the absence of holes therein and because the merger decreased the total number of arcs. The fusible element is, accordingly, provided with the opportunity to interrupt lower phase-to-ground fault currents by generating a more slowly increasing, lower back voltage (rather than one that is "too high"), thus preventing the back voltage of the fuse from exceeding a selected value, such as the sparkover value of the surge arrestors.
It has been found that, in some circumstances, the more gradual burn-back rate which occurs after group arc merger may still increase "too quickly" and may generate too high a back voltage when interrupting phase-to-ground fault currents. That is, the burn-back rate of the ribbon between the hole groups by the merged arcs may be too fast, thereby generating sufficient back voltage to cause sparkover of surge arrestors. Accordingly, a general object of the present invention is to provide an improved fusible element of the type set forth in U.S. Pat. No. 4,359,708 which effectively interrupts fault currents driven by both higher phase-to-phase voltages and lower phase-to-ground voltages, while limiting, with more assurance, the back voltage generated by the fuse during interruption of fault currents at the lower phase-to-ground voltage.